| Summary: | Results are reported for the BS0->μ+μ− branching fraction and effective lifetime and from a search for the decay B0 -> μ+μ−. The analysis uses a data sample of proton-proton collisions accumulated by the CMS experiment in 2011, 2012, and 2016, with center-of-mass energies (integrated luminosities) of 7 TeV (5 fb−1), 8 TeV (20 fb−1), and 13 TeV (36 fb−1). The branching fractions are determined by measuring event yields relative to B+ → J/ψK+ decays (with J/ψ → μ+μ−), which results in the reduction of many of the systematic uncertainties. The decay BS0 -> μ+μ− is observed with a significance of 5.6 standard deviations. No significant excess is observed for the decay B0 -> μ+μ−, and an upper limit of ℬ(B0 → μ+μ−) < 3.6 × 10−10 is obtained at 95% confidence level. The BS0 -> μ+μ− effective lifetime is measured to be 1.70 (+0.61/-0.44) ps. These results are consistent with standard model predictions.
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